Peter's Perfect - Reengineering 321 Formula to the Ultimate Coco Experience

Well folks, I’m back at it again. This time more determined than ever. We can rebuild it, stronger, faster, better than ever. Armed with my trusty HydroBuddy, we have the technology.

With my first (sort of… there have been more) failure under my belt, and 3 weeks until I kickoff my next grow, I’m back to the books and ready to reboot. So, fellow Jacks/Peters 321 abusers, let’s try this again.

At this point, you might be asking, why mess with a good thing? OCD my friends… that and the unwillingness to pay more money for the same chemicals with fancy marketing. You read that correctly, I’m cheap, but mama didn’t raise no fool. Here’s what I think can be improved:

Same NPK in veg and flower? We can certainly do better than that…

  • Too much K in veg.
  • Not enough N in veg.
  • Not enough K in flower
  • Too much N in flower?
  • Not enough S in flower
  • Not engineered for coco

Let’s lay some ground rules, and find some targets for the next experiment. First thing is first, and that is to answer the question: why did I fail last time? Well things went well for a while, but eventually symptoms showed problems on the K/Ca/Mg front. In part, I think I wasn’t getting enough runoff from my feeds, but even then, the buildup wasn’t terrible when I measured after a flush. After doing a bunch of reading today, I found something that clicked. The answer comes once again from… MULDERS CHART!!!

Mulder’s chart is a chemistry-oriented approach to analyzing nutrient interactions. It tells us that all nutrients are inextricably linked. One cannot simply increase the presence of one nutrient, without affecting potentially several others.

While I knew that K, Ca and Mg all interact, because they are all positive ions, I didn’t have the frame of reference to know what ratios are reasonable. Today, I found that link and it made sense.

…Applying too much calcium and magnesium can cause a potassium deficiency; the K/Ca and K/Mg ratio should always be kept above 2 (but below 10, since too much K can hinder the absorption of calcium and magnesium). Too much potassium can also prevent the absorption of certain micro-elements, such as zinc. It is particularly important to take account of this interaction when using very hard water with a high calcium and magnesium content.

I think that will provide the pivotal frame of reference that I was missing. Adding this constraint will certainly make our designer nutrient more challenging to accomplish, but we must press on.

So here’s a few things I know about nutrients and coco, that will help us determine how to design a custom Jacks/Peters-based formula that beats 321.

  • New coco substrates hold significant quantities of K
  • New coco substrates replace K in bonding sites in favor of Ca and Mg (initially depletes Ca/Mg, adds K)
  • Our K:Ca ratio needs to be no less than 2:1 and no more than 10:1
  • Our Ca:Mg ratio needs to be around 2:1 to 4:1
  • Cannabis (and all plants) use more Ca than P, so set Ca > P
  • Sulfur may be beneficial in flower for the production of terpenes
  • Cannabis needs more N in veg and more K in flower

According to my HydroBuddy, the 321 formula comes out to a ratio of 3-2-5-2.5-1.25 N-P-K-Ca-Mg.

For those who haven’t used 321, it’s 3.6g Peters 5-11-26, 2.4g Calcinit, 1.2g Epsom.

Already, we can see that this hits the 2:1 K:Ca ratio right on the head, and also a 2:1 Ca:Mg ratio. Target ratios right where we expected them. So lets go about trying to figure out a veg formula.

I’ve done a sampling of the internet in days passed. Some very knowledgeable growers (f@tman?) have suggested that 3-1-2 is ideal for veg. I think that this is not a bad starting point. We have lower K initially, to deal with the problem of K already in the substrate, and more N.

Let’s add in our Ca and Mg to this at 2:1. Since we know that coco will deplete this initially, I think we want Ca as high as possible.

That would put us at:

3-1-2-1-0.5 N-P-K-Ca-Mg – however with early life Ca deficiencies, I think I want more.

A small tweak allows us to pull up the Ca:

So… how do we achieve this. Well, one idea is to drop the Epsom in favor of Mag Nitrate. That will boost our N, and drop S, which we aren’t too concerned about right now. Unfortunately, hydro buddy says this doesn’t get us close enough. We could do it with ammonium nitrate, but because of it’s explosive potential, it’s not available in the US to the general public.

If we drop N/K relative to P, let’s see if we can make it work:


For this to happen, we will need to use ammonium sulfate. There’s a lot of bad press about ammonium, but in the right ratios, it may turn out to be beneficial for this reason:

Knowing that ammoniacal nitrogen drops pH three times faster than nitrate nitrogen raises it, the water-soluble grower can mix ammonium sulfate with calcium nitrate and/or potassium nitrate to achieve a 1:3 ratio of ammonium to nitrate nitrogen. As plants uptake these two nitrogen forms, the opposing pH pressures cancel each other out, resulting in low to no pressure on pH.

Interesting… as long as we keep it in ratio, it might actually help balance out the nitrates, which tend to increase pH at the root zone.

This roughly works out, close to targets. So, for the veg formula, I’ve removed epsom, and added ammonium sulfate and magnesium nitrate. Additionally, I’ll use TM-7 to make up the micronutrient difference for any reduction in the amount of Peter’s used.

Veg Formula

Here’s the mix:


    Peters 5-11-26        -- 24g
    Calcium Nitrate       -- 16g


    Magnesium Nitrate  -- 8g
    Ammonium Sulfate  -- 6g
    X TM-7 (removed)

And here’s the results:

Not bad – we hit our 3:1 NO3:NH4, our 2:1 K:Ca and pretty close to a 2.5-1-2.5 ratio. This is pretty reasonable. I’m glad to see that peters and calcium nitrate balanced out to 3:2 as well. That’s inspiring. Without running some plants, I can’t think of any way to improve it, so I think this is a reasonable formula to test.

Next post, I’ll try to figure out a formula for bloom.


Well that all looks confusing as hell! How does anyone keep it all straight ? Are there any plant whisper’s on this OG forum? Lol


:joy: Well I’m finding out why people pay for premixed nutrients… It’s a whole research project in itself, and you’re really paying for someone to figure it out.

My hope is that I can come up with something that’s essentially the base nutrients of the Jacks/Peters 321 formula + a few other cheap nutrients, to make something that totally kicks ass in coco. I’ve seen enough minor problems over the past 2 years of using it, that I’m dead set on cracking this nut. People use it in coco (myself included), but there’s good reasons that coco-specific nutrients exist. And Jacks/Peters 321 was developed for hydroponics, and not for coco.

Unfortunately, I’ve found out that I have to spend a lot more time learning about nutrient interactions than I ever wanted. But hey, if it works out in the end and benefits some other people as well, then what the hell. It’s better than watching tv. :nerd:


Ok, now for flower…

At this point, the calcium will have saturated in the medium, so we can probably relax a bit on the K:Ca ratio. We are likely to want more K, and more S at this stage. So I’m going to add a 3rd chemical for this purpose – potassium sulfate. By allowing the K:Ca ratio to drop from 2:1 gradually to ~3:1, the plant should increasingly uptake K when it needs it, but probably not so much that we’ll hit deficiencies.

This one is easy, and we will use potassium sulfate as our “knob”. My plan is to start around week 3 of flower with the “bloom” formula at around a 2-1-3-1.3-0.65 ratio and gradually increase potassium sulfate 1g at a time until the ratio is 2-1-4-1.3-0.65.

To do this I will start in week 2 with this formula (approx 10-15gal, adjust for desired EC):

Flower Formula


    Peters 5-11-26        -- 36g
    Calcium Nitrate       -- 24g
    Epsom                    -- 10g


    Ammonium Sulfate -- 7.5g
    Magnesium Nitrate -- 3g
    Potassium Sulfate  -- 3.5g  --> ending as high as 10g

Here’s the starting ratios at week 2:

Perfect! I still managed to keep the NO3:NH4 ratio, so pH balance should hold steady in transition. I think we can drop the TM-7. It’s not needed here, and unlikely to be a problem in the veg formula. The sulfur is really cranked up with the potassium sulfate and ammonium sulfate. I don’t think this will cause problems, but if it does, we can reduce the ammonium sulfate, reduce the magnesium sulfate and increase the magnesium nitrate.

The plan now is to amend the 321 formula with only 3 extra chemicals. If it works, you’ll probably want ~3-5lbs each of magnesium nitrate, ammonium sulfate and potassium sulfate per 25lbs of peters formula.


Some links:

I have this mag nitrate/oxide… I will try it, but I’m not sure about the MgO component. Probably not absorbed without chelation. The question answered says 16% MgO, but 11-0-0 provides the same amount of N :thinking:

Prefer this one:


Check out maxibloom,calimagic,and Amour Si. Those npk should be close to what your looking for…


You sir, are the MAN! It is so awesome of you to do all the work. My brain starts hurting after a while lol! I sure hope you crack this nut too, as I’m going crazy looking for something that kicks ass in coco that I can mix up myself. I’ve tried so many different formulas, I’ve have recently just settled on the kiss method with maxibloom and it has been the best for me so far. I know I can find a more complete formula and I hope this one your working on is a winner. I am thinking about adding maxi grow for more of the micros for the time being at maybe 2.5g of maxi grow plus 2.5 of maxibloom to a gal with a little silica added and see how that goes for now.


I’ll might check those out if I can’t make this work. I have about half my body weight in Peter’s though. :sweat_smile:

Man, me too. Over time, I’ve come to realize that you can just casually modify formulas and get good results. You really need to do the work and understand everything to get there… and even then, you still have to run the experiment, identify what went wrong and adjust again. It’s a long process.

I feel like the 321 formula isn’t bad. But if you’ve ever used it in hydro, it’s a thing of beauty. But it was also designed for hydro “Jacks/Peter’s hydroponic special”, and coco is NOT hydro. Lot’s of misinformation out there about coco being inert, blah blah. We inherently know it’s not true or we wouldn’t say things like “don’t run pure water through coco” or “coco needs to be buffered”. If you need to buffer it, it ain’t inert. There’s a lot of K wrapped up in there at the start, and that’s a knife that cuts twice.

Hydro formulas do work with coco… but I think they’re not inside the sweet spot. So when you run 100 plants, and some are more Ca hungry or some like more N, you’re more likely to see those problems when you use a hydro formula than if you use something designed for coco. I want to figure out how to widen the margins to success, and I think it’s just some small tweaks on the standard 321 formula.


Just throwing out chemical npk numbers. These people here gots game…


You said it brother! I was going to give jacks a go here in the next run, but I’m having success with the maxi bloom and I’m nervous to switch lol! I was also going to order mega crop cause I like all the goodies in it, but don’t wanna start messing with to many formulas at once again. I’m doing a lot of reading on mixing your own salts and formulas, but I get lost easy. That’s why I’m grateful for super smart growers like you to help me understand it better.


Very cool products. I like their schedule… it gives me ideas. Interesting that the MKP boost happens only in week 3-4

Here’s their ratios (without MKP boost) @ 100g each of ChemGro420, Calcium Nitrate and Epsom:

Looks like 1-1-2, 10:1 NO3:NH4, 2:1 Ca:Mg (most of the grow it’s more like 3:1 though with Epsom at 60g) and 2:1 K:Ca.

Then they boost with MKP, and here’s what that looks like at week 4 (300g Chemgro420, 300g Calcium Nitrate, 200g Epsom, 100g MKP)

NPK 2-4-5, then 3:1 K:Ca to really amp up the absorption of K.

This is probably a really killer schedule for hydro setups. I think it still might have a few weaknesses for coco, especially the high amounts of K in veg having similar issues to the Peter’s formula. I like their ideas around using MKP at weeks 3-4. I might incorporate that into my experiment…

They have a pretty decent amount of sulfur. I’d still like to try more, especially at the end of the cycle. I think MKP is probably nice, right where they add it. Might pull potassium sulfate out for weeks 3-4 and back in through the end.


Did quite a bit more reading and found some very enlightening information from this post on thcfarmer:

Specifically this quote in there:

Coco Nutrition

We have seen that coco substrate contains naturally high levels of potassium (K) and therefore a nutrient formulated for coir will have lower potassium (K) levels than a standard nutrient. Other than this we have also seen that coir naturally contains sulphates and therefore a nutrient developed for coir would contain less S than a standard nutrient (or so this should be the case – at least one “hydro” manufacturer has been known to pass off standard nutrients labelled as coco formulations).

I think I want to be cautious here about adding more sulfur, and perhaps I need to reconsider if potassium sulfate is a good additive to use in bloom. Now, I am thinking of removing potassium sulfate and ammonium sulfate. The substitution of magnesium nitrate for magnesium sulfate in veg seems very beneficial.

I can’t help but think that a more idea 321 for coco simply using magnesium nitrate as the “1” will be the KISS answer. It achieves boosting N and provides Mg, without adding S.

Here’s a literal 321 formula with magnesium nitrate rather than magnesium sulfate:

So we have about a 6-3-8 ratio, Ca:Mg stays in check at 2:1, K:Ca also 2:1. I think the only thing I’d consider is about 2g of Ammonium Sulfate, and perhaps a 7-10 day bloom boost around weeks 3/4.

I like where this is heading. I think good things are in store for us.


I’ve been added K2NO3 to my mix for a couple grows now. I dig it.

My issue is keeping enough Ca and not too much N mid to late flower.

Someone had mentioned a chelated calcium nitrate that’s 5-0-0 and still 19% Ca. Was very pricey though. About 4x the cost of regular CaNO3


Sulfur is usually the wildcard in formulation as plants tend to tolerate sulfur over a relatively wide range.
What does your sulfur PPM look like with the sulfates?

edit: I looked up the rule of thumb for sulfur from Harley Smith. He notes, between 40min-500max ppm with the suggested maximum not exceeding 400 ppm. If that’s useful.

Cool project, by the way.


Yeah that’s what I’m concerned about now – too much N late flower. I think swapping magnesium sulfate for magnesium nitrate is going to be nearly ideal for veg. However, I wonder if your calcium problems are actually related to too much K as well. Too much K will suppress Ca, Mg. Another idea is to reduce Mg to a 1:3 ratio after the PK boost at week 3/4. This might be a simple solution, since Ca/Mg compete as well. It will also lower Mg. I think that one could go as low as 4g, keeping all other parts of the 321 formula unchanged. Alternatively, N could go even lower by changing that from magnesium nitrate to epsom at the end.

With a 321 formula using magnesium nitrate, it is 38ppm, compared to 60 for Mg and 127 for Ca. If adding 2g additional ammonium sulfate, it is 51ppm.

Some numbers for coco, suggest that it has quite a bit inherent. Testing around >1000ppm, though I’m not sure how/when this would become available to the plant.


So for reference we can consider CANNA’s line developed for coco:

Their part A&B is supposedly 5-4-3 NPK (veg). Then starting in flower, they add approximately an equal part of canna boost at 0-1-1. So that becomes 5-5-4. Then as soon as small buds form, it looks like they add a VERY heavy dose of pk in rough numbers somewhere around 5-13-13. Then back to 5-5-4.


I didn’t even think of that. I know the 321 is a happy 4:2:1 K:Ca:Mg but adding more K in the form of potassium sulfate and adding less Ca with less calcium nitrate would definitely swing things.

I don’t have hydrobuddy(computer died back in May) if you wanna run these #'s. 2.7g mix 1.2g calcium nitrate and 0.5g potassium sulfate. This is all per gal.

I may pick up that chelated calcium nitrate and use that week 3 and on. Be nice to be able to increase the calcium nitrate to compensate for additional K during bulk and finish without going too crazy on the N

But I’ll be following this thread closely to see if there’s a cheaper alternative.

Curious what 2.7g mix 1.8g chelated calcinit (5-0-0-19) 0.9g MgSO4 and 0.5g potassium sulfate looks like for finish

And 2.7g mix 1.8g chelated calcinit 0.9g MgSO4 and 0.5g MKP looks like for bulk

Also what those two scenarios looks like keeping K to Ca 2:1 what calcium nitrate #/gal need to be to get there


I’ll run those numbers for you when I get home from work. I think that’s the critical point is keeping K:Ca maxed out pretty much the whole time at 2:1, strictly because the Coco medium is sucking up Ca and leaching out the K.

I also think that Canna achieves lower amounts of Ca by using a fairly low value of K. But since our K is fixed by the 5-11-26 formula, we would have to reduce that, which I feel causes a cascade of corrections that need to be applied to compensate. It would kind of defeat the purpose by adding half a dozen other nutrients into the mix.

The real mystery is pointed out in that thc farmer thread. How does P > Ca work out and why?

I think for first experiments I would like to start out with simple changes and see what happens. I will probably use magnesium nitrate instead of Epsom through 2 weeks of flower, then add MKP at week 3 for 1 week, then switch to Epsom for the rest of the grow to reduce N. My only concern is that this will still have too much N at the end. If it does, then your idea of chelated calcium might do the trick.


Have you put in calcium chloride? I had heard chloride is pretty safe as far as interactions similar to sulfur.

Expensive at $406/50# bag


So this is 2.7g Peter’s, 1.2g Cal Nitrate and 0.5g Potassium Sulfate

You have a 4:1 K:Ca ratio, and that might be causing the calcium defs. Approx 1:1:3 NPK.

Here’s your 2nd formula 2.7g Peters, 1.8 Chelated Cal, 0.9 Epsom and 0.5g potassium sulfate:

I think you don’t need the potassium sulfate in that formula, or use maybe half that amount. With the potassium sulfate you have 3:1 K:Ca, without you have 2:1 (and 1:1.3:3 NPK).

Here’s the last one: 2.7g Peters, 1.8g Chelated Cal, 0.9 Epsom and 0.5g MKP